Furuduy Discuss. Chern. SOC., 1989, 88, 113-122 Electroactive Films from Poly( ethylene oxide)-Sodium Iodide Complexes with Tetracyanoquinodimethan Jamil A. Siddiqui and Peter V. Wright* School of Materials, University of Shefield, Northumberland Rd, Shefield S10 2TZ The preparation of conducting polymer films from oriented PEO-NaI by exposure to I2 and charge-transfer exchange with tetracyanoquinodimethan (TCNQ) is described. The PEO-NaI had stoichiometries x = [EO]/[NaI] = 3-6. Gravimetric, X-ray and i.r. analysis suggests that crystalline complexes of PEO-Na13 and PEO- NaTCNQ have been prepared having conductivities of lop4 S cm-' and lop's cm-I, respectively. Exchange with TCNQ forms bilayers of PEO-NaTCNQ/PEO-NaI. Exposure of the x = 3 bilayer to I2 vapour brings about a transformation of the exchanged layer (7-10 pm in thickness) to a microcomposite of a more conductive complex salt within a PEO matrix having a conductivity 1 S cm-'.The apparent conductivity of these films falls to a minimum ( S cm-I) at x = 4, but an increase in conductivity of at least two orders of magnitude is observed for x = 5 films which have much thinner exchanged layers and a more integrated structure than the x = 3 materials. The conductivity of the PEO-NaTCNQ/I, layer is found to depend on the 'gate' potential applied between the layer and a silver-epoxy electrode in contact with the PEO-NaI, layer in a triode configuration. A number of years ago, some semicrystalline complexes of poly(ethy1ene oxide) (PEO) with a variety of lithium, sodium, potassium and ammonium salts were prepared in our laboratory.' These materials have been extensively studied by virtue of their significant ionic conductivity through the amorphous phase' and their potential as thin-film 'polymer electrolytes' in batteries incorporating alkali metal electrodes3 or in other devices.Considerable effort has therefore been devoted to the synthesis of various amorphous copolymers or networks of PEO with the aim of maximising ionic cond~ctivity.~ However, we have also begun an investigation of the electrical and thermo-optical properties of the crystalline PEO complexes.5-* Alongside the inorganic salts, a wide variety of complexes with planar organic anions such as phenolates, naphtholates, carboxylates and sulphonates may also be prepared.In the cases of sodium and lithium salt complexes, these anions are considered to stack alongside a helical PEO-cation adduct having a stoichiometry of 1 mol of salt per 3 ethylene oxide units (x = 3). Such a structural model is inferred from unit-cell data9,"' and the detailed crystallographic investigations of PEO- NaI and PEO- NaSCN complexes by Chatani and coworkers. ''*I1 The latter have shown that the PEO adopts a 2/1 helix enclosing two cations per fibre repeat (7.98 and 7.19 A, respectively). In some of the organic anion complexes, self- organised macrodomain textures have been observed6.' which undergo mesomorphic transformations at ca. 60 "C to microdomain or spherulitic textures before melting to isotropic phases at temperatures up to 200 "C. In this paper, we report on the preparation, characterisation and electrical properties of films of crystalline PEO- NaI complexes which have undergone a charge-transfer exchange reaction with tetracyanoquinodimethan (TCNQ) in hydrocarbon solvent and of a crystalline complex of PEO-sodium polyiodide.PEO-NaI +TC"Q PEO-NaTCNQ+$I, (1) 113114 Electroactive Films from EO Complexes PEO-NaI++n12 PEO-NaI, In an initial brief report of the first of these materials6 [eqn (l)] it was shown that the conductivity is maximised if the PEO-NaI precursor film is oriented. In this work we also report studies of PEO-NaTCNQ post-treated with iodine giving films or fibres of enhanced conductivity. Experimental PEO-NaI was prepared by dissolving PEO of molecular weight 5 x lo6 (BDH) and NaI in molar ratios x = [ethylene oxide units]/[NaI] = 3-6 in methanol to give polymer concentrations of ca.10%. Oriented films of PEO-NaI were prepared by slowly shearing small aliquots of this solution between glass surfaces using a motorised rig. The solvent was rapidly removed in a stream of warm air to give an oriented crystalline film of uniform texture and thickness 10-30 pm. Silvery-grey PEO-NaI, films were prepared by exposing oriented PEO-NaI to iodine vapour in a desiccator for 3 days. The charge-transfer reaction with TCNQ was carried out by immersing the slide in a 2% solution of TCNQ in sodium-dried mesitylene. After 36 h the purple film on the slide was removed, washed in dry mesitylene and stored in a desiccator. PEO-NaTCNQ films on glass slides were exposed to I2 vapour by placing in a desiccator in the presence of I2 crystals at atmospheric pressure for 5 days.Following this treatment, the films took on a dull-grey appearance and the I2 was then removed from the desiccator. After leaving to stand in air in the desiccator for a further 3 days, the exchanged surface became a green-gold colour appearing highly lustrous when traces of surface crystalline material were removed. PEO-NaTCNQ fibres were prepared by dry-spinning oriented PEO-NaI fibres from the methanolic solution of the complex which was delivered from a hypodermic syringe onto a rotating wire frame. The frame was then immersed in TCNQ-mesitylene as before. X-Ray fibre photographs were obtained using a flat-plate camera and a Phillips X-ray generator.Attenuated total reflectance infrared spectroscopy was carried out using a Perkin- Elmer 683 spectrometer fitted with a Specac ATR accessory incorporating a KRS-5 crystal in contact with the exchanged surface of the film. Scanning electron micrographs were obtained using a Cambridge 600 instrument. Polarised light optical micrographs were taken using a Polyvar Met microscope. Conductivity measurements were performed using a Solartron 1250 frequency- response analyser and a 1286 Electrochemical interface employing four-probe (surface) techniques and two- or three-electrode cells. The measurements were carried out under vacuum in the presence of P205 as desiccant. Results and Discussion Representative gravimetric data of molar compositions of precursor films on glass slides and exchanged and iodine-doped materials are given in table 1.As the first row of the table shows, the oriented PEO-NaI film with x = 3 absorbs 1 mol of I2 per mol of NaI. As shown by the diffractometer tracings in fig. 1 the film is transformed to give a new crystalline complex which is apparently PEO-NaI, . Differential thermal analysis of this material indicates a melting endotherm at ca. 145 "C (PEO-NaI melts at approximately 190 OC'**) and also an endotherm at 50-70 "C, as commonly observed in PEO-alkali-metal salt complexes.' The latter may arise from uncomplexed PEO or low melting complexedFaraday Discuss. Chem. Soc., 1989, Vol. 88 Plate 1. Scanning electron micrograph of a fracture surface profile of a PEO-NaTCNQ (upper laqer)/PEO-NaI bilayer mounted on a glass slide.Sample was pre-cooled in liquid nitrogen before fracture. Scale bar = 10 pm. Plate 2. Transmission optical micrograph through a PEO-NaTCNQ/PEO-NaI bilayer (x = 3) between crossed polars. Overall thickness is ca. 5 pm. Scale bar = 20 pm. Plate 3. Wide-angle X-ray fibre pattern of PEO-NaTCNQ with fibre axis vertical. J. A. Siddiqui and P. V. Wright (Facing p. 115)J. A. Siddiqui and P. V. Wright 115 Table 1. Molar compositions of films” A B C P( EO), NaI gain after exchange iodine uptake X / lo4 moI / lo5 mol TCNQ /lo4 moI B / A C I A 3 3.5 3 1.2 5 1.3 6 1 .o - 6.7 1.6 0.7 7.2 - 2.1 3.9 0.56 3.3 6.0 0.12 4.6 4.4 0.07 4.5 “ Representative data from a series of experiments as films mounted on microscopic slides all of the same surface area (2.5 cm x 7.5 cm).n n 1 I I I I I 30 20 10 2 e i o Fig. 1. Wide-angle X-ray diffractometer tracings (Cu K , radiation) for ( a ) PEO-NaI, and ( b ) PEO-NaI. The data of the second row of table 1 indicate that in films of 15-20 pm thickness with stoichiometry x = 3 ca. half of the iodide ion was exchanged with TCNQ. Thicker films contained lower proportions of TCNQ. The estimates given in column B of table 1 were made assuming that all the iodine liberated on the right-hand side of eqn ( 1 ) was retained within the film. The compositions of the exchanged PEO-NaTCNQ (x = 3) films suggested that the gravimetric analysis is corroborated by scanning electron microscopy. An SEM photo- graph of the fracture surface of an exchanged film with stoichiometry x = 3 mounted on a glass slide is shown in plate 1.The section is apparently a bilayer of an overall thickness of ca. 15 pm, consisting of an exchanged layer of PEO-NaTCNQ and a layer116 L 000 Electroactive Films from EO Complexes 3000 2000 1000 wavenumber/cm-' Fig. 2. Attenuated total reflectance infrared spectra of (a) PEO, ( b ) PEO-NaTCNQ (x =3), ( c ) NaTCNQ. of unpenetrated PEO- NaI, each layer being of approximately equal thickness. More extensive experiments on films of various thicknesses suggest that, under the preparative conditions described above, the exchanged layer is 7-10 pm in thickness. The last two rows of table 1 also show that a significantly lower proportion of the available iodide ions was exchanged with TCNQ in the films with x = 5 and 6.This apparently indicates reduced swelling of these film surfaces by the reaction medium. Plate 2 is an optical micrograph of a film of PEO-NaTCNQ ( x = 3 ) between crossed polarisers with the orientation direction at 45 O to the plane of polarisation. The film is approximately 5 pm in thickness and is presumed to be almost fully exchanged. The crystallites are ca. 0.9 pm in thickness along the orientation direction. Although some surface crystals are present there are transparent regions where phase-separated crystals of NaTCNQ are smaller than may be observed using visible light if not present as a chemical complex with the PEO. No further structural detail on the crystallites could be discerned using carbon-replica transmission electron microscopy but the structural discontinuities along the draw direction between crystallites were seen to be voids bridged by fibrils.Attenuated total reflectance infrared spectra of a film of PEO, a film of PEO- NaTCNQ and a compressed pellet of NaTCNQ are shown in fig. 2. The spectrum of the film includes the bands which are characteristic of the -CH2- (ca. 2900 cm-') and -C-0- (ca. 1100 cm-') vibrations of the polyether. Since the depth of penetration of the surface by radiation of wavelength A is 0.29A using the KRS-5 c r y ~ t a l , ' ~ the spectra show that the PEO is present in the exchanged layer within 1 pm of the surface. The shift to lower frequency of the -C-0- vibration in PEO following exchange isJ. A. Siddiqui and P. V. Wright 117 observed in other PEO complexes and is consistent with coordination of the oxygens to a cation.An X-ray fibre photograph of PEO-NaTCNQ (x=3) is shown in plate 3. The strongest spots give a pattern of well defined layer lines corresponding to a fibre-repeat distance of 6.6 8, (kO.1 A). However, a more faint layer line may also be discerned, which may suggest the presence of a second oriented phase with repeat distance 11.6 A. Neither repeat distance corresponds to that expected for pure PEO crystals (fibre repeat = 19.3 8,) and a comparison of the scattering angles of hkO diffractions along the layer lines with diffractometer tracings of NaTCNQ powder indicates significant differen- ces in the patterns. A more detailed analysis of the fibre pattern is awaited. Meanwhile, it is tentatively assumed that the principal fibre pattern represents a complexed form of PEO-NaTCNQ. According to the structural investigations of the crystalline phases of PEO-NaI and PEO-NaSCN (form I) by Chatani and coworkers,11,12 the PEO chain adopts a 2/1 helix with six ethylene oxide units per helical repeat.These investigations and the unit-cell measurements of Parker et allo and Hibma' suggest a general model for PEO-Na' complexes in which the PEO-cation 2/1 helical adduct is able to adjust, at least over the range 7.19-8.40 A in order to accommodate various inorganic anions of different dimensions. This spacing should also comfortably accommodate the thickness of two planar aromatic anions stacked alongside the helix. Siddiqui" has determined fibre repeat distances for PEO-sodium phenolate and PEO-sodium 4-phenyl phenolate complexes (which also have crystalline stoichiometry x = 3) and has determined fibre repeats of 7.1 8, (*0.1 A) in each case which is ca.twice the thickness of the phenyl ring. These data are thus consistent with a schematic model consisting of a PEO-cation helical adduct with anions stacked alongside. The interplanar distance of ca. 3.3 8, between TCNQ molecules, which is indicated by the X-ray fibre pattern, is in the range which has been observed16 in crystals of TCNQ salts which display high electronic conductivity. However, highest conductivities are generally observed in complex salts of mixed valence incorporating uncharged TCNQ molecules, which provide vacancies for electron tran~fer.'~ The proportion of 'neutral TCNQ' in TCNQ salts has been estimated" from the higher energy peak of the split band corresponding to the -C_N stretching vibration at ca.2200 cm-'. Fig. 4(6) and (c) suggests the presence of both charged and neutral forms in PEO-NaTCNQ and in our sample of NaTCNQ. In films of PEO-NaTCNQ doped with I2 (x = 3 ) it is clear from optical reflection microscopy that, following the transformation to the green-gold material, there is considerable phase separation of needle-shaped salt crystals of long dimension, ca. 1-3 pm. Such phase separation is also indicated by the similarity between wide angle X-ray diffraction patterns of the exchanged surface of these films and NaTCNQ salt similarly treated with 12. Consideration of other iodine complexes of TCNQ suggests that the complex salt may have the formula NaTCNQ( 13)0.33.Microscopic investigation of the films indicates that the molecular orientation of the precursor PEO-NaI films, though retained following the exchange reaction, is partially lost after doping with 12. The data in table 1 indicate that the overall absorption of iodine by exchanged films is somewhat greater than in the case of the unexchanged PEO-NaI. Since less iodine per mole of NaI should be required by phase-separated microcrystals of NaTCNQ( 13)o.33 than by the non-exchanged PEO-NaI layer, it seems likely that the extra I2 has been taken up by the phase-separated PEO in the exchanged layer, forming an amorphous matrix for the microcrystals. Greater I2 uptake by PEO not engaged in crystalline complex formation with NaI thus accounts for the high absorption by materials with x > 3.Absorption of I2 in excess of that required for conversion to crystalline PEO-Na13 is presumably limited by the Coulombic energy of the ionic lattice.118 h Electroactive Films from EO Complexes -2 - 4 2.6 2 . 8 3.0 3 . 2 3 . 4 lo3 K/ T Fig. 3. log a, versus reciprocal temperature (a, = bulk conductivity) plots for surface measure- ments on films of PEO complexes. For films exchanged with TCNQ, (T, was calculated assuming that the thickness of the exchanged layer is 10 pm (see text). H, Oriented PEO-NaTCNQ-I,/PEO- NaI, bilayer, x = 3; 0, oriented PEO-NaTCNQ/PEO-NaI bilayer, x = 3; 0, oriented PEO- NaTCNQ/PEO-NaI bilayer, x = 4; A, non-oriented PEO-NaTCNQ/PEO-NaI bilayer, x = 3; 0, oriented PEO-NaI,, x = 3. ( - * .) PEO-NaI, x = 3;13 (- - -) PEO-NaI, x = 4' (measurements through compressed pellets). Fig. 3 shows log( conductivity) versus reciprocal temperature plots for bulk and surface measurements of various films. The bulk conductivity is given by a, = a, / t, where a, is the surface conductivity and t is the film thickness. In the case of exchanged films with surface layers of PEO-NaTCNQ and PEO-NaTCNQ-I,, the thickness t was assumed to be 10 pm, as indicated by the gravimetric and SEM analysis described above. The surface conductivity of PEO-NaTCNQ layers is significantly greater for the oriented films than for those prepared from non-oriented, spherulitic precursor films. The conductivity of the oriented PEO-NaTCNQ layer is almost two orders of magnitude greater than that of compressed pellets of NaTCNQ salt (2 x S cm-' at 20°C).Furthermore, the results for the PEO-NaTCNQ film reported here are ca. five-fold larger than reported previously.' The improvement may be attributed to the more uniform machine-oriented texture of the precursor film in the present work and the use of mesitylene rather than toluene as a medium for the exchange reaction. Although TCNQ salts are generally more conductive along the direction of the stack, as noted previously6 there appears to be little anisotropy of conduction in the plane of the drawn films, the conductivities perpendicular to draw being slightly the greater. This is perhaps a consequence of the morphology as shown in plate 2, which features structural discontinuities between crystallites along the draw direction.The voltage-current slow linear sweep data for these materials appear to be linear and reversible throughout theJ. A. Siddiqui and P. V. Wright 119 temperature range ambient to 120°C for both x = 3 and x = 4 testifying to the essential electronic mechanism of conduction with little contribution from the underlying PEO- NaI material. The lower surface conductivity of the PEO-NaTCNQ x = 4 material may thus be attributed to reduced intercrystalline contacts than for x = 3. The crystalline PEO-NaI, material gives an approximately linear plot in log,, 0, vs. 1/T from ambient temperature up to ca. 90°C. Above this temperature, the fall in conductivity may be attributed to loss of Iz.However, a broad endotherm in thermal analysis traces at 120-150°C apparently represents melting of the new complex and indicates that a substantial proportion of iodine is retained in the material at higher temperatures. Shriver and coworkers*' have measured the conductivities of NaI, complexes ( n = 1-9) in an amorphous ethoxy-based comb polymer, polybis( 2-( 2-methoxyethoxy))ethoxylphosphazene (MEEP). They report the conduc- tivity of MEEP,NaI, at 30°C to be 3 x lOP4S cm-' which is approximately the same magnitude as that indicated in fig. 6 for the crystalline PEO-NaI, complex. Using linear sweep voltammetry and complex impedance analysis, Shriver and coworkers concluded that the conduction proceeds by an essentially ionic transport process in their amorphous systems.However, slow linear-sweep data for the crystalline system studied in the present work are essentially linear throughout the temperature range displaying only slight irreversibility at higher temperatures, and a.c. measurements indicate that the conductivity is frequency independent up to lo4 Hz in some experiments. Thus, although some ionic mobility in less-organised regions of the material may account for the slight irreversibility observed in some samples, the conduction process is apparently dominated by electronic transfer in the crystalline complex. The surface conductivities of iodine-doped PEO-NaTCNQ layers at ambient tem- peratures are more than two orders of magnitude greater than those either of the undoped films or of compressed pellets of the iodine-doped free salt which was estimated to be 3 x lo-, S cm-'.The enhanced conductivity in the complexed salt may be attributed to the distribution of the formal negative charge between the TCNQ and iodine atoms. The decrease in conductivity at temperatures above 60 "C coincides with loss of iodine in the exchanged layer and the restoration of the purple colour of the undoped material. However, in these materials some non-ohmic behaviour is apparent from the linear sweep data (as in fig. 5, later). This behaviour could arise, for example, from polyiodide mobility in the supposed amorphous polymer matrix surrounding the phase-separated complex salt in the exchanged layer. The presence of this phase could account for the greater volatility of iodine in this material when compared with the more tightly bound halogen in crystalline PEO-NaI,. Fig.4 shows log,,u, versus stoichiometry, x, for PEO-NaTCNQ and PEO- NaTCNQ-I, at ambient temperature. The uo (apparent) values obtained from four- probe surface measurements on exchanged films were calculated using the overall film thicknesses. As table 1 shows, in films prepared from PEO-NaI with x > 4, significantly lower proportions of available iodide ion exchange with TCNQ than in the case of x = 3. Thus the surface layers vary in thickness with x and are much thinner for x = 5 and 6, exhibiting significantly greater transmission of visible light in optical microscopy than the materials with thicker exchanged layers. The a,(real) values for the surface layers, which have been calculated using the layer thicknesses estimated from gravimetric data, are also plotted in fig.4. In the case of x=5, the estimated thickness of the exchanged layer (ca. 1 pm) was also confirmed using scanning electron microscopy. The conductivity of the PEO-NaTCNQ films appears to decrease with x, becoming too small to measure by the 4-probe surface technique when x>4. The underlying PEO-NaI material clearly makes an insignificant contribution to the conductivity at ambient temperature. However, the iodine-doped material appears to display a minimum in conductivity at x = 4. The conductivity normal to the exchanged, iodine-doped films is, very much120 Electroactive Films from EO Complexes r I 1 I 3 4 5 6 stoichiometry, x Fig. 4. log (apparent conductivity) versus stoichiometry x = [EO]/[Na+] for bilayer films of PEO complexes.0, Surface measurements on the TCNQ-exchanged layer of PEO-NaTCNQ-IJ PEO- Na13 bilayers; a, measurements normal to the latter bilayers; 0, surface measurements on exchanged layers of PEO-NaTCNQ/ PEO- NaI bilayers. (-) Apparent conductivities calcu- lated using the overall bilayer thicknesses; (- - -) real conductivities calculated using exchanged layer thicknesses. less dependent on x than is the corresponding surface conductivity and is of a magnitude (ca. lo-’) consistent with that to be expected for the PEO-NaI, layer. Thus, the variations in surface conductivity with x may be attributed to corresponding variations in the structure of the surfaces. The reason for the significant increase in conductivity at x = 5 is not clear and these systems require further investigation.Inspection by optical reflection microscopy of the x = 5 and 6 materials indicate regions of variable composition but of a finer texture than the phase-separated crystals within the exchanged layer of the x = 3 material. A more integrated structure, either on the molecular level with mixed TCNQ-iodide complexes or at the interfaces of different complexed phases, is perhaps indicated for x > 4. If an electrode of silver-epoxy is placed in contact with the PEO-NaI, layer (an inert electrode remaining in contact with the TCNQ-exchanged layer) linear-sweep voltammetry normal to the surface of the film gives tracings such as that of fig. 5(a). The anodic sweep of the inert electrode meets the voltage axis at +0.67 V.This lies close to the difference between the standard electrode potentials I,+2e- S 31-; +0.5338 V (3) AgI+e- $ Ag+I-; -0.1519 V. (4) This suggests that the pronounced asymmetry in the tracing (which is not observed with PEO-NaI layers in contact with a silver electrode) arises from the rapid formation of a passivating layer ofAgI when the potential on the inert electrode falls below +Oh7 V. Similar traces were observed at sweep rates up to 6 V s-’ and voltage ranges up to A12 V. The system thus behaves as an electrochemical diode. Fig. 5( 6) shows the characteristics of a triode arrangement in which a film of PEO-NaTCNQ-I, is sandwiched between a pair of gold ‘drain’ electrodes in contact with the exchanged layer and a silver-epoxy reference electrode in contact with the PEO-NaI, layer.A gap of ca. 250 pm separatedJ. A. Siddiqui and P. V . Wright 121 1 - V vs. Ag 4 vg = 2 . 0 v 5 2.0-- drain voltage, V,/V Fig. 5. (a) Linear voltage-current sweep data normal to a PEO-NaTCNQ-I,/PEO-NaI, bilayer (x = 3) having a silver-epoxy electrode in contact with the PEO-Na13 layer and a gold electrode in contact with the exchanged layer. (- - -) 200 mV s-'; (-) 50 mV s-'. (b) Triode characteris- tics of a bilayer film (x = 3) between gold (solid shading) and silver (crossed hatching) electrodes. The diagonal hatching is the PEO-NaTCNQ-I, layer and the unmarked layer is PEO-Na13. Film thickness is ca. 30 pm. Separation of gold electrodes is ca. 250 pm. Fig. 6. Schematic diagram of a proposed structure for PEO-NaTCNQ-12, x = 6.Smaller circles (+) are sodium ions; larger circles (-) are polyiodide ions; rectangles are TCNQ anions or neutral molecules and the solid line represents the PEO 2, helix.122 Electroactive Films from EO Complexes the gold electrodes which were ca. 1 cm2 in total area. With a ‘gate’ potential of 0.7 V applied across the bilayer in the conductive mode (Ag negative), the conductivity in the exchanged layer increases to ca. three times that of the open-circuit conductivity. Gate potentials in excess of 0.7 V causes excessive current leakage across the bilayer. The increase in ‘drain’ current for gate potential s0.7 V presumably arises from an increase in the population of neutral TCNQ molecules and/or iodine atoms in the exchanged layer so creating vacancies for electron transfer.The lowering of conductivity by the suppression of such vacancies on reversing the gate potential was not observed. This presumably arises from the presence of the passivating layer on the silver electrode which drastically reduces the conductivity across the film. Further investigations of the structure and mechanisms of conduction in these materials are in progress. References 1 D. E. Fenton, J. M. Parker and P. V. Wright, Polymer, 1973, 14, 589. 2 P. V. Wright, Br. Polym. J., 1975, 7, 319. 3 M. B. Armand, J. M. Chabagno and M. Duclot, in Fast Zon Transport in Solids, ed. P. Vashisha, J. N. 4 J. M. G. Cowie, in Polymer Electrolyte Reviews 1, ed. J. R. MacCallum and C. A. Vincent (Elsevier, 5 J. A. Siddiqui and P. V. Wright, Polymer Commun., 1987, 28, 7. 6 J. A. Siddiqui and P. V. Wright, Polymer Commun., 1987, 28, 89. 7 B. Mussarat, K. Conheeney, J. A. Siddiqui and P. V. Wright, Br. Polym. J., 1988, 20, 293. 8 P. W. Wright, in Polymer Electrolyte Reviews 2, ed. J. R. MacCallum and C. A. Vincent (Elsevier, 9 T. Hibma, Solid Sfate Zonics, 1983, 9/10, 1101. Mundy and G. K. 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